Measurement of an electron beam extracted from the plasma edge cathode

The plasma edge cathode concept can produce high electron current densities while avoiding or delaying plasma closure of the extraction gap of an electron source. A plasma beam is generated by surface flashover from a spark plug. An obstacle partially intercepts the emitted plasma jet at a distance of about 150 mm from the source and causes a stationary transverse plasma boundary from which electrons are extracted. The plasma should experience negligible external electric forces as long as the extraction is space charge limited. Thus, it is theoretically possible to avoid the plasma closure of the extraction gap and to obtain an extracted electron current density over 100 A/cm/sup 2/. Experimental results partially support this theoretical prediction. The pepper-pot method was used to measure the emittance of the large-area anode of 42.4 cm/sup 2/. A range of the extraction voltage was kept from 6 kV to 23 kV. The measured electron-beam area was 4/spl times/4 cm. The effective emittance was on the order of 5/spl times/10/sup -3/ mrad with some shots of 10/sup -3/ mrad. The electron beam densities of the large-area anode of (42.4 cm/sup 2/) were measured. At charging extraction-voltages over 17 kV, the Faraday cup detected an electron current density with a duration of approximately 2 /spl mu/s. The average electron density is 5-20 A/cm/sup 2/ with the peaks up to 45 A/cm/sup 2/. A computer code calculated the projection densities of J(x,x´), and J(y,y´) from digitized electron pepper-pot-images of the 4 x 4 cm effective electron-beam area.